The present invention relates to improved displays for indicating instruments, and more particularly to a display system for an electrical instrument in which raised numerals are illuminated by reflected light.
With the advent of miniature electric batteries and electrically-driven watch movements it has become possible to construct electric wristwatches and the like of conveniently small size. Further, the introduction of light emitting diodes, or LED's as they are commonly known, has allowed watch manufacturers and others to provide illuminated alphanumeric displays which can be operated by the same electric battery used to drive the instrument.
Several limitations inhere in the conventional LED-type of display such as used in conventional watches and calculators. For one thing, LED displays usually require a relatively high current flow, which places an undue strain upon the small batteries which are used. Further, LED-type of readouts are substantially planar, and are often difficult to read from an oblique angle due to the parallax effect which arises. Finally, the conventional, red LED displays are difficult to read in sunlight. This causes a user to keep the display energized longer than desirable, increasing the drain on the instrument battery.
Although alternative types of electrically-illuminated displays are known, prior art displays commonly require more illumination than it is feasible to drive with present-day miniature batteries. Various types of "light pipes" and related light-transmitting glass and plastic members are known; however, it will be recognized that the inherent losses occasioned by the transmission of light through an extended body of glass or plastic further reduce the illumination available, and accordingly require a more powerful source of illumination.
Conventional approaches to illuminating indicia by reflected light usually utilize a bulb or the like which shines upon the indicia to illuminate them. Again, however, this approach is wasteful of light energy, and aside from requiring an inordinate amount of electrical energy is necessarily large and bulky. This renders it unsuitable for use in miniature readouts such as are necessary for watches and the like.
Finally, conventional illuminated readouts of the low-power type, such as LED displays, cannot be read unless they are illuminated inasmuch as the display and background elements are practically indistinguishable when the unit is not energized.
Accordingly, it will be appreciated that it would be highly desirable to construct an illuminated miniature display which is small in size, and which requires only a relatively small amount of electric power for energization.
It is therefore an object of the present invention to provide an improved, illuminated alphanumeric display.
Another object is to provide an improved illuminatable display for electric wristwatches and the like.
Yet another object is to provide a display which can be illuminated under low-light conditions, yet is easily readable under ambient light, without other illumination.